To make beer, brewers use water and barley to create a sweetened liquid (called the wort), which they flavour with hops, then ferment with yeast. The basic process may be simple but the execution is highly sophisticated. The most important stages are malting, brewing and fermentation, followed by maturation, filtering and bottling. During fermentation, extracted carbohydrates in the wort are converted by yeast into alcohol and carbon dioxide, while new yeast cells are formed. The maturation step is an important aspect of the beer brewing process; maturation is required to create beer with mature and consistent flavour. During primary fermentation, one of the first phases is yeast growth. During this phase, as the yeast cells multiply, all biochemical activities including amino acid synthesis are “switched on”. In the biochemical pathway of valine synthesis (an amino acid), alpha-acetolactate is formed in excess quantities and secreted by the yeast cells into the wort. In the wort outside the yeast cells the alpha-acetolactate is converted chemically by decarboxylation to a diketone, diacetyl; the conversion of alpha acetolactate to diacetyl is a chemical reaction controlled by the pH, temperature and the redox state of the beer.
Many diketones, including diacetyl, are strong flavour components and have a very low taste threshold; diacetyl has a flavour threshold of about 0.10-0.15 mg/l and has a strong buttery or butterscotch flavour, which is a characteristic of “green” or “young” or immature beer. In order to balance the flavour of beer after primary fermentation, secondary fermentation or other forms of maturation are utilised to reduce the concentration of diacetyl below that of the human taste threshold. The classic method of maturation involves “secondary fermentation”. During this phase, the yeast cells consume diacetyl and enzymatically reduce diacetyl to yield acetoin. Acetoin has a significantly higher taste threshold, i.e. about 8-20 mg/l, than diacetyl.
The reaction rate of diacetyl to acetoin is much faster than the reaction rate of acetolactate to diacetyl under standard brewing temperatures and conditions. In order to prevent the formation of diacetyl in the maturation stage, it is necessary to limit the amount of diacetyl precursors (including alpha acetolactate) present as the beer leaves the maturation stage. If this is not accomplished, “potential” diacetyl remains which, after the maturation stage, can alter the flavour of the beer.
Secondary fermentation is a lengthy process, which can typically last several days or more and is conducted as a “batch” process. Commercial brewers have sought methods which “mature” beer faster, cheaper and more efficiently. Among the prior art methods utilized is a diacetyl “rest”. In this process, the green beer is held at a temperature of 12-18° C. for a few days to about a week following primary fermentation. This “rest” allows all or substantially all of the alpha-acetolactate to be secreted by the yeast and reduced to diacetyl, which is subsequently enzymatically reduced by yeast to acetoin during maturation.
Utilization of a continuous maturation process could potentially increase the efficiency of and reduce the brewing time and cost for the production of beer.
In the production of yeast fermented beverages, such as beer, continuous fermentation, including continuous maturation, can offer a number of significant advantages, including:                higher productivity and lower investment: equipment can be operated for prolonged periods of time under full load, meaning that for equal production volume smaller vessels are needed than in a batch process;        constant and better quality: process is easier to control due to possibility of adapting process parameters to local and instantaneous requirements and because steady-state-conditions are much more stable;        high hygienic standard: continuous process is operated in a closed system.        less energy: energy consumption is evenly spread, without major use peaks; and        less labour: operation of continuous process requires less attention        less standstill and cleaning: continuous process can be operated at much longer run lengths than batch processes.        
These potential advantages of continuous fermentation have been recognised by the brewing industry a long time ago. Accordingly, many attempts have been made to design continuous fermentation processes that do indeed deliver those potential benefits.
U.S. Pat. No. 3,234,026 describes a method for the continuous fermentation of brewer's wort to produce a potable beer, said method comprising: maintaining in separate vessels, separate yeast propagation and product formation process stages; maintaining aerobic conditions in the yeast propagating stage; maintaining substantially anaerobic conditions in the product formation stage; continuously introducing brewer's wort to the yeast propagation stage and maintaining yeast propagation therein; continuously passing effluent from the yeast propagation stage to the product formation stage; separating yeast from effluent of the product formation stage and maintaining the yeast concentration in the product formation stage at an artificially high level by re-introducing a portion of the separated yeast to the product formation stage. The US patent teaches to separate the yeast from the fermented liquid in a separating vessel in which the yeast quickly flocculates and settles out to the bottom of said vessel. The US patent further teaches to pass the clarified fermented liquid by pipeline to a heat exchanger to reduce the temperature and subsequently to a holding vessel where the liquid may be held for a predetermined time and withdrawn for storage and finishing.
GB-B 1 300 116 describes a continuous fermentation process which comprises feeding unfermented wort to the lower part of a first fermentation vessel, passing the wort upwardly in the vessel through a zone containing an essentially stationary homogeneous mass of yeast, removing partially fermented wort containing a small proportion of yeast from the upper part of the first vessel, passing the partially fermented wort and yeast to an intermediate point in a second vessel, where further fermentation may take place and removing the fermented wort from the upper part of the second vessel, and continuously or intermittently removing settled yeast from the bottom of the second vessel. The British patent describes an embodiment in which fermented wort containing suspended yeast is removed from the second vessel and passed into a settling tank where the suspended yeast settles and a clarified fermented wort is removed from the upper part of the settling vessel.
EP-A 0 022 613 describes a method of continuous fermentation in which a carbohydrate solution is fed continuously in a fermentation zone containing substantially homogeneously distributed yeast and carbohydrate solution, a proportion of the fermenting liquid continuously passing to a pressurised settling tank, yeast depleted liquid being withdrawn from the upper part of the settling tank, yeast enriched liquid being withdrawn from the lower part of said tank and returned to the fermentation zone.
DE-A 100 03 155 describes a discontinuous method for accelerated production of beer in which green beer is produced with the help of immobilised yeast. The green beer so produced contains some non-immobilised yeast which is removed through sedimentation in a sequence of two decanters that are being cooled to 0-10° C.